linux-kernel - [PATCH v2] tty: serial_core: Fix serial console crash on port shutdown

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Date:   Tue, 18 Oct 2016 18:35:40 +0200
From:   Geert Uytterhoeven <geert+renesas@...der.be>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>
Cc:     Niklas Söderlund 
        <niklas.soderlund+renesas@...natech.se>,
        Rob Herring <robh@...nel.org>,
        Peter Hurley <peter@...leysoftware.com>,
        Alan Cox <alan@...ux.intel.com>, linux-serial@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Geert Uytterhoeven <geert+renesas@...der.be>
Subject: [PATCH v2] tty: serial_core: Fix serial console crash on port shutdown

The port->console flag is always false, as uart_console() is called
before the serial console has been registered.

Hence for a serial port used as the console, uart_tty_port_shutdown()
will still be called when userspace closes the port, powering it down.
This may lead to a system lock up when the serial console driver writes
to the serial port's registers.

To fix this, move the setting of port->console after the call to
uart_configure_port(), which registers the serial console.

Fixes: 761ed4a94582ab29 ("tty: serial_core: convert uart_close to use tty_port_close")
Reported-by: Niklas Söderlund <niklas.soderlund+renesas@...natech.se>
Signed-off-by: Geert Uytterhoeven <geert+renesas@...der.be>
Acked-by: Rob Herring <robh@...nel.org>
---
v2:
  - Add Acked-by,
  - Reworded one-line summary (incl. magic word "fix").

Can easily be reproduced on Renesas boards:
  1. With systemd: hangs during boot,
  2. Without systemd: "telinit n" to switch to a runlevel that doesn't
     run a getty on ttySC0, and trigger kernel console output => hang.
---
 drivers/tty/serial/serial_core.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index 6e4f63627479db8d..ce8899c13af3d97f 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -2746,8 +2746,6 @@ int uart_add_one_port(struct uart_driver *drv, struct uart_port *uport)
 	uport->cons = drv->cons;
 	uport->minor = drv->tty_driver->minor_start + uport->line;
 
-	port->console = uart_console(uport);
-
 	/*
 	 * If this port is a console, then the spinlock is already
 	 * initialised.
@@ -2761,6 +2759,8 @@ int uart_add_one_port(struct uart_driver *drv, struct uart_port *uport)
 
 	uart_configure_port(drv, state, uport);
 
+	port->console = uart_console(uport);
+
 	num_groups = 2;
 	if (uport->attr_group)
 		num_groups++;
-- 
1.9.1